"It's been known for some time that people who are obese tend to have higher levels of pain, generally speaking," said investigator Andrew Schrepf.

"But the assumption has always been the pain is going to be in the knees, hips and lower back -- parts of the body that are weight-bearing," said Schrepf, of the University of Michigan Chronic Pain and Fatigue Research Center.

Schrepf and his colleagues assigned 123 obese people to a low-calorie liquid diet. They also directed them to gradually increase physical activity.

The study found that losing 10 percent of body weight over 12 weeks helped reduce pain not only in expected locations, such as the knees and hips, but also in the abdomen, arm, chest and jaw.

Weight loss also led to better mental health, improved thinking and more energy, according to the study.

"We know when people lose a lot of weight they tend to feel better. But astonishingly, no one ever looked at where in the body the pain gets better," Schrepf said in a university news release.

The findings suggest that joints aren't the only conduit of body pain, according to the researchers.

"What we think that means is this process of losing weight may be affecting the central mechanisms of pain control related to the brain and spinal cord," Schrepf said.

He and his colleagues now want to find out why losing 10 percent of body weight was the point at which obese people showed improvement in pain and other areas.

"Some of your earliest weight loss isn't all fat; it could be water," Schrepf said. "Somewhere around 10 percent we're reaching some kind of critical mass, but it's hard to know exactly what that means."

The findings were published recently in the Journal of Pain.

More information

The U.S. Centers for Disease Control and Prevention has more on weight.